Re: how to read/write object from/to binary file?

[cblake]

While doofenstein makes some good points especially about fragility with regard 
to adding new fields and old data files, there is some real efficiency charm to 
a "just memory map and go" pure binary native data approach. It's not always 
fun to be parsing data files again and again. So, I would also add another way 
to read the data. Note that you should probably use the `{.packed.}` pragma, 
note the use of `ptr UncheckedArray` which should really alert the reader to be 
careful, note the commented out inference of the number of objects in the file. 
My example code assumes you wrote out at least two objects from code like you 
had above (with a `{.packed.}` pragma). It will probably crash if the file is 
too short, etc. Just to give you a flavor. 


import memfiles
type
  MyType* {.packed.} = object
somefield*: array[10, int16]
  MyTypes* = ptr UncheckedArray[MyType]
var t: MyType
var f = memfiles.open("/tmp/tmp.mytype")
# infer n object from file size (may need packed); Do own range checking!
# let n = f.size / sizeof(MyType)
let myobs = cast[MyTypes](f.mem)
echo myobs[0].somefield[1]
echo myobs[1].somefield[0]

Re: how to read/write object from/to binary file?

[doofenstein]

Because it's very dangerous.

Simply having a compiler which puts structures in another layout or adding a 
field to the object is enough to render all your previously generated files 
useless.

If you just want to save and load data, I would rather serialise 
it([https://nim-lang.org/docs/marshal.html](https://nim-lang.org/docs/marshal.html)
 ). If you want something more compact then e.g. JSON or YAML take a look at 
msgpack: 
[https://nimble.directory/pkg/msgpack4nim](https://nimble.directory/pkg/msgpack4nim)
 (note: I haven't used msgpack with Nim, but I had good experiences with it 
with other programming languages)

Re: how to read/write object from/to binary file?

[luntik2012]

Thank you, it works. It strange that there is no possibility to make some cast 
from string of seq[byte] directly to object

Re: how to read/write object from/to binary file?

[c0ntribut0r]

Have a look at this:


import streams
type
  MyType* = object
somefield*: array[10, int16]
var t1, t2: MyType
t1.somefield[0] = 1
t2.somefield[0] = 2


var f = newFileStream("/tmp/tmp.mytype", fmWrite)
if not f.isNil:
  f.write t1
  f.write t2
f.flush

var r1, r2: MyType
var f2 = newFileStream("/tmp/tmp.mytype", fmRead)
discard f2.readData(r1.addr, r1.sizeof)
discard f2.readData(r2.addr, r2.sizeof)
echo r1.somefield
echo r2.somefield

Re: how to read/write object from/to binary file?

[luntik2012]

Sorry, it works correctly. But how to read it back? File contains several 
instances of MyType, so I need to take subpart of file. I've tried this 


let data = readFile("/tmp/tmp.mytype")
let ololo = cast[MyType](data[0 .. 

Re: how to read/write object from/to binary file?

[luntik2012]

I do 


t.somefield[0] = 255
t.somefield[1] = 255



to check

how to read/write object from/to binary file?

[luntik2012]

I can do something like this: 


type
  MyType* = object
somefield*: array[10, int16]
var t: MyType
var f = newFileStream("/tmp/tmp.mytype", fmWrite)
if not fff.isNil:
  f.write t


but result file contains garbage, size of file is 20 bytes.

So question is how to read/write object from/to file?